Flotation of continuous strip by pressure pads directing gas streams against the strip is a mature art as shown by the basic patents incorporated by reference herein related to work performed in the early 60's (Conceptually "strip" and "web" are used interchangeably herein, while "strip" may conventionally refer to metal and "web" may conventionally refer to non-metallic materials, the system disclosed herein will float both metallic and non-metallic material.) Flotation is frequently explained by referring to certain basic physical principles. Three commonly used principles to explain web flotation are the Bernoulli principle, the Coanda effect and of course the law of conservation of momentum. Combinations of the above principles are sometimes used to explain the operation of nozzle employed for web flotation.
Since these are all well known principles, they are cited in the Background to establish a reference basis. The Bernoulli principle simply stated is that the static pressure of a stream of fluid is reduced as its speed of flow is increased. The applications and illustrations of the previous statement are virtually unlimited. Some of the more commonly used examples are: the Venturi meter, the atomizing sprayer, and the plates suspended by air from a nozzle above the plate. The Coanda effect is the tendency of a gas or liquid coming out of a jet to attach itself to an adjacent wall contour when the wall direction of curvature is away from the axis of the jet. Conservation of momentum is a fundamental law of mechanics stating that a force is produced which is proportional to the rate of change of momentum. The force on a web can be predicted by considering the direction and momentum of the fluid leaving the nozzle and the change in direction and momentum of the fluid in contact with the web.
Again, the flotation of a web or strip by air or some other gas is accomplished by considering all the forces acting at any part of the web: i.e. the force of gravity, tension and forces exerted by the air or gas supplied to one or both sides of the web. The position of the web relative to the nozzle is where all the forces are in equilibrium. Other important considerations relating to the understanding of the operation of flotation nozzles include, (1) entrainment of secondary air in the area of the nozzle supplementing the primary air from the nozzle and (2) the return air arrangement which affects floating height and heating of the web.
The design principles discussed generally above have been utilized to developed various nozzle or pressure pad designs for floating or suspending strip on a gas cushion. When a heat treatment process is involved, the object becomes not only to stably float the continuous web (and optionally move it in a controlled manner from point A to point B) but also to impinge the web with a gas at high velocity and at a different temperature (either hotter or colder) to effect rapid heat transfer therewith i.e., effecting a high, mass heat transfer between gas and strip.
Despite claims made by conventional pressure pad manufacturers, it has nevertheless been experienced that impacting or impinging the strip with high velocity jet flow caused the strip (especially thin gauge strip which for aluminum is less than 0.040") to flutter and that such fluttering, became so severe that the strip actually contacted the pressure pad above or below (or sometimes both) the strip with the result that strip marking occurred. Because the strip is thin gauge, reducing the velocity of the gas jet will produce (as predicted by design principles discussed above), a strip which is capable of being stably floated between the pads without significant flutter. However, the mass flow of heat treating gas through the pad or the nozzle is correspondingly reduced, which means that either the strip line speed will have to be reduced to allow the pressure pad adequate time to effect sufficient heat transfer with the strip (either heating or cooling), or alternatively, additional pressure pads have to be installed thus increasing the length of the flotation system to obtain the desired strip temperature dictated by the heat treat process. A study or survey of existing pressure pad designs, including those of the current assignee of this invention, failed to disclose a pad design which, for light gauge strip (especially strip which may vary in gauge thickness lengthwise), could stably float the strip while simultaneously effecting high heat transfer rates with the strip.